1. Field of Endeavor
The present invention relates to the field of gas turbines. It refers to a method of operating a gas turbine plant and to a gas turbine plant useful for carrying out the method.
2. Brief Description of the Related Art
A gas turbine plant 10, such as is reproduced by way of example in FIG. 1, includes a compressor 11 for the compression of combustion air sucked in from the surroundings via a compressor inlet 18, a combustion chamber 15, in which fuel supplied via a fuel supply 20 is burnt by means of the compressed combustion air, and a following turbine 12, in which the hot gas coming from the combustion chamber 15 is expanded so as to perform work. The turbine 12, via a shaft 14, drives the compressor 11 and a generator 13 which generates electrical power and discharges it via a transformer 16 to a network (not illustrated).
Such a gas turbine plant 10 is operated at very high temperatures in order to achieve good efficiency. There have hitherto been no suitable measuring instruments for the temperatures occurring at the exit of the combustion chamber 15 and at the turbine inlet 21. These temperatures are nevertheless important for regulating the gas turbine plant. For a long time, therefore, other parameters of the gas turbine plant, such as, for example, the exhaust gas temperature downstream of the turbine, the temperature at the compressor outlet and various pressures or pressure conditions, which can be measured reliably, have been measured, and, for example, the temperature at the exit of the combustion chamber is determined indirectly from these parameters (see, for example, EP-A2-1 231 369).
It became apparent, in this context, that the composition of the gas, in particular the water content, in the working medium flowing through the gas turbine has to be taken into account in determining the combustion chamber exit temperature. If a specific fixed gas composition is assumed, the algorithm used for determining the temperature can be designed accordingly. Problems arise, however, when the composition of the gas, in particular the water content, deviates considerably during the operation for which the algorithm for temperature determination is designed. Whereas the natural fluctuations in the atmospheric humidity of the sucked-in ambient air are comparatively low and therefore cause fewer problems, for example, the introduction of water into the intake tract gives rise to considerable deviations in the water content within the framework of the power-increasing methods designated as high fogging or wet compression.
In this case, it is found that the gas composition is highly dependent on the composition of the fuel or of the fuel quantity, particularly in the combustion of lean gases (synthesis gas, industrial gases, integrated coal gasification).
It has therefore already been proposed, in the patent application initially mentioned, to measure the supply of water on the intake side of the compressor and to take these values into account in determining the combustion chamber exit temperature. In a parallel patent application (US-A1-2004/0076218, now U.S. Pat. No. 6,805,483), at least the title refers to the fact that the water content of the fuel may also be taken into account with a correcting effect in the determination of the combustion chamber exit temperature.
By determining the water taken up together with the intake air and/or with the fuel into the working medium, some effects can be taken into account in temperature determination. There are, however, further types of operation of gas turbine plants which may have effects on temperature determination in terms of the water content in the working medium. What is critical in this case is the steam content in the exhaust gas of the turbine. If this content deviates considerably from the value on which the algorithm for temperature determination has been based, the operating point and the temperature drop across the turbine are displaced according to the known relation for isentropic expansion
            T      ⁢                          ⁢      6              T      ⁢                          ⁢      7        =            (                        p          ⁢                                          ⁢          6                          p          ⁢                                          ⁢          7                    )                      K        -        1            κ      with the temperature T6 and the pressure p6 at the turbine inlet (measurement point M6 in FIG. 1), with the temperature T7 and the pressure p7 at the turbine outlet (measurement point M7 in FIG. 1) and with the ratio “κ” of the specific heats.
Typical examples of modes of operation of the gas turbine plant with the injection of water or steam are as follows:                power increase                    injection of water or steam into the combustion chamber;            injection of water or steam into the cooling air system (see, for example the publication EP-A2-0 995 891);                        exhaust gas check (NOx, etc.)                    injection of water or steam into the combustion chamber;                        for gas turbine plants with quencher                    injection of water into the low-pressure and high-pressure cooling air coolers and consequently into the cooling air system;                        intake air cooling                    evaporation cooler;            fogging;            high fogging;                        water content as combustion product.        